I been tossing around the idea of building a "cost effective" controller that can grow as your tank does. It will be loosely based on 2 units, both based on the Arduino Due. The hardware will be custom tailored for harsh and wet environments ( AKA IP67 / IP68 ) thus driving the price up a bit, however its insurance in the long run if a splash or drip wont kill the system.

The Display Unit
It will be based on a 7" 800x480 capacitive touch display ( a 5" version coming later on ), will be running an Atmel SAM3X ( Arduino Due ) in 16 Bit serial mode with some added bits to make the system work seamless and smooth. An internal Micro SD card slot will be added for storage for most of the code and data for GUI as well as an RTC will be added with battery backup. The unit will be sealed and a there will be only a single connector. A subminiature 12 pin waterproof Hirose connector will be used to break out I2C, USB and the Power of board. The programming can be done on your desktop as the unit will come with the cable to the Breakout Box as well as a programming cable that will plug right into your PC and allow you to program it via the Arduino IDE as an Arduino DUE through the Native Programming Port of the SAM3x. There will be ICSP headers on board just in case something happens during programming. This way if something happens during programming, you don't end up with a bricked unit.
Overall goal is to keep this unit thin and light as possible... with that being said I would like to keep it thinner than 13mm ( 1/2" ).

The Main Unit or Breakout Box
This unit will house most of the controls.
There will be support for the Atlas Scientific Stamps, which allows readings PH, OPR, Dissolved Oxygen as well as Conductivity and 2 spare spots fo future add-on's. Will add 4 temperature probes for good measure and redundancy, also a backup RTC on this board in case the display is unplugged or just not present all together, the system will continue to function unaffected. Another general 6 inputs will allow you to hook up any switch ( either float or for the door as most of us have small kids at home and we dont want them poking around the sump ).
All the accessories will be I2C enabled devices, mainly to simplify wiring, and it cant get simpler than 4 Wires ( V+, GND, SDA, SCL )

Smart Power Bar
Each of its outlets can be assigned to either turn on or off, dependent on your set parameters. Also, there will be a few versions available:
The 8R, 8SS or 8H is 8 outlet with either a Mechanical Relay ( R ), Solid State Relay ( SS ) and Hybrid ( H ) units. There will also going to be a 10 outlet version with 6 mechanical relays and 4 solid state relays. The Solid State Relays will have an option to either work as ON / OFF or PWM.

4/8/12/24 CH High Current / Voltage LED Driver
This unit is a modular unit as well that can grow as your tank does ( very rarely we start with tank we want, bust mostly we start small and "upgrade" from there ). I have designed this unit based on TI's LM3463 LED Driver. This chip can be controlled either via PWM, I2C or as simple as a Potentiometer as it accepts analog input ( some of us don't want all the bells and whistles but want something rather simple ). The MOSFET's are Enabling Modules ( EM ) on their own, since they carry their own sense resistor.

When the EM is unplugged from the board the channel gets disabled. This way if you want to add channels is as simple as plugging in another EM.
The PWM Control Board is based on the PCA9685 ( a 16 CH PWM driver ) that has a 6 hardware address pins that allow up to 62 of these devices on the bus simultaneously!
Again... this is all about flexibility and growth.

It does seem like a bit of an equipment Sprawl, however I DO NOT like mixing low voltage and high voltage equipment.
All the modules I/O's will be safeguarded by optocouplers as well as proper grounding techniques. In case of an accident a fried Optocoupler is cheaper to replace!
The whole system will run off a smallish power supply 15W ( 5V 3A ) or smaller depending on how many modules your going to decide to run ( power bars will have their own internal power supply )

Here's the nice part...

This project is almost in prototype phase!
The software will be open source! The code is free to use and tailor to your own needs as long as you own the product and GUI will be the same. Arduino being an open source allows tons of flexibility, ease of use and with TONS of support GLOBALLY !! Not to mention I will be seeking the Arduino At Heart certification for the Display and the Breakout Box

Here's where you guys come in... think of this as a suggestion box.
If it's feasible its possibility you may see it come to fruition in the near future

I have been playing with a similar idea using the design form a guy on another form. Are you thinking about selling pre built or just sparking up a project?

Robo-Tank ?

Mine will be built, no assembly required.
Once everything is finalized i will order the 3/8's black acrylic for the housing and will have them machined. The front and the rear will be sealed using a large O-Ring in machined groove.
I will have a site up in a few weeks as well as a support / user forum.

I built my own a few years ago. Wow, it was a programming adventure finding different ways to measure everything i wanted. The things i found most useful where mostly stuff that default controllers don't generally come with.

I really enjoyed being able to track my PH/Temp/Etc from a webpage. I used a Google pages website locked to my account, this would be harder to do now since i'm pretty sure Google scrapped their pages. I also used pachube, now Xively for all data logging. When you're board waiting at the dentist office it's amazing how much you'll look into that day 3 weeks ago when your tank got hotter.

Also, temp probes are dirt cheap, i had 4 and averaged the temps across them and checked if within limits of the others to check for bad probes. I had lots of heat shutoff's and didn't want my tank reacting to a failed temp probe.

The thing i liked the most was color indexing. I'm old school and still use Halides, you can track color shift very easily now so it's well worth looking into.

Alerts are fairly easy to set up through a twitter feed, i had any alarms sent to my twitter. Any measured value outside of range, sump overflow sensor, sump level sensor, herbie level sensor, and a few others i'm probably missing.

I also liked variable day light, most of you fancy LED people have this built in now. I calculated sunrise/sunset times on the gold coast, this gave me short days in summer(their winter) and long days in winter, which helps with heating issues.

The only issue I've ever had with this system is my power supply. Get a decent regulated power supply, especially if you're using mechanical relays. I learned this after frying my first arduino.

I built my own a few years ago. Wow, it was a programming adventure finding different ways to measure everything i wanted. The things i found most useful where mostly stuff that default controllers don't generally come with.

I really enjoyed being able to track my PH/Temp/Etc from a webpage. I used a Google pages website locked to my account, this would be harder to do now since i'm pretty sure Google scrapped their pages. I also used pachube, now Xively for all data logging. When you're board waiting at the dentist office it's amazing how much you'll look into that day 3 weeks ago when your tank got hotter.

Also, temp probes are dirt cheap, i had 4 and averaged the temps across them and checked if within limits of the others to check for bad probes. I had lots of heat shutoff's and didn't want my tank reacting to a failed temp probe.

The thing i liked the most was color indexing. I'm old school and still use Halides, you can track color shift very easily now so it's well worth looking into.

Alerts are fairly easy to set up through a twitter feed, i had any alarms sent to my twitter. Any measured value outside of range, sump overflow sensor, sump level sensor, herbie level sensor, and a few others i'm probably missing.

I also liked variable day light, most of you fancy LED people have this built in now. I calculated sunrise/sunset times on the gold coast, this gave me short days in summer(their winter) and long days in winter, which helps with heating issues.

The only issue I've ever had with this system is my power supply. Get a decent regulated power supply, especially if you're using mechanical relays. I learned this after frying my first arduino.

Most of the Heavy draw items like the power bars will have their own supplies for the relays. The relays will be in turn triggered by a FET which is triggered by the PCA9685. going this route is great as there are 62 address combos that you can choose for your smartbar, LED driver or whatever output device your using.
Also forgot to mention that since DC pumps are hitting the market there will be a 4CH DC pump controller on the horizon as well.

The hardware to me is the easy part!
The software on the other hand... lets just say im a bit rusty LOL

Also, temp probes are dirt cheap, i had 4 and averaged the temps across them and checked if within limits of the others to check for bad probes. I had lots of heat shutoff's and didn't want my tank reacting to a failed temp probe.

As far as the temp probes go I'm planning on using the DS18B20 based units as the 1 wire serial bus is simple and the accuracy for the price is hard to beat ( 5 pieces with a 2m cable run for about $10 ). Also because of the single wire bus you can have as many sensors as you want for redundancy and it all boils down to how much room are you going to spare in the case.

I'm not a controller person, so I have no input. For me, the only type of "controller" I'm interested in is more of a "monitor" that alerts me via text or email when things are wacky. What kind of input are you looking for?